The present invention relates to switching means for electrical circuits, and more particularly to an improved control system for limiting current flow through the actuating coil of a contactor.
It has long been recognized that electromagnetically-operated switches of the type generally termed contactors exhibit disparate current requirements for actuation and for continued operation after closure. In particular, when such a device is deenergized, an armature is normally spring-biased away from the core of an electromagnetic coil. Operation of the device is dependent upon the magnetic attraction between the core and the armature such that when current is applied to the coil magnetic flux is produced in the core, which in turn attracts the armature. The armature, in moving toward the coil core, forces the contacts of the device closed (or open, if the device is of the normally-closed type.).
Owing to commonly-understood magnetic phenomena, a substantially larger current is required to move the armature to a closed position than to maintain it there. In general, this is because the magnetic circuit exhibits a higher reluctance when open, due to the air gap; and because it is desired to rapidly operate the contacts in order to avoid arcing or burning. Frequently the coils are formed of conductor which is heavy enough to accomodate the peak, or maximum, current requirements which occur only during closure. In simple systems a constant-value voltage is applied to the coil, giving rise to a current which is sufficient to operate or "pull in" the armature. However, such current continues to flow after the armature is pulled in, even though it is far greater than what is required to maintain the armature in its closed or "holding" position. In such a case the coil must be formed of a substantially larger conductor than otherwise would be required for the "holding" current, and further must be capable of withstanding the additional thermal loads produced by the higher current. It is obviously uneconomical to operate a coil in such a fashion, i.e. to provide a constant current flow of a magnitude which is needed only for pull-in conditions. Accordingly, numerous efforts have been made to construct contactors in such a manner that a relatively high current flow is provided for pull-in operation and a lower, more economical current for "holding" operation.
One popular approach has been the so called "long wipe" contact arrangement in which an auxiliary, sliding contact is provided, and arranged in such a manner that they are engaged only during the pull-in period, i.e., when the contacts are open (or closed, in a normally-open type of contactor). A resistor is connected in series with the contactor coil, and the auxiliary or long-wipe contacts used to shunt the resistor. In this manner the resistor is effectively short-circuited during the time when the contacts are in their normal, unactuated position, but when the contactor is actuated the auxiliary contacts disengage so that the resistor limits current flow after the contactor attains its fully actuated position.
In recent years various attempts have been made to provide a still more effective circuit for economizing current levels in contactor coils. Typically, such efforts involve the use of sensitive switches or semiconductors to switch current between a low impedence and a high impedence path for effecting high and low current flows, respectively. The placing of impedences in circuit with a coil, however, is only a partial solution inasmuch as the losses in the resistance substantially offset the energy which is conserved in decreasing current flow. It will therefore be understood that it would be desirable to provide an improved current control system for a contactor or the like which allows sufficient initial current flow for pulling in the contactor, while regulating current flow to a lower value sufficient for the subsequent holding condition.
It is accordingly an object of the present invention to provide an improved current control for actuating a contactor or the like.
Another object of the invention is to provide a time ratio control system for metering current to the energizing coil of an electromagnetically-operated switch.
Yet another object is to afford an economical solid-state circuit for minimizing coil current in a contactor.
Another object is to provide an improved current ecomonizer circuit for use with a contactor or the like.